Oil operating temperature and efficiencies are very important to the designers, builders, and user of equipment which employ worm gearing. On a relative basis, a higher percentage efficiency rating for a lubricant results in more power or torque being transmitted through a subject gearbox. Since more power is being transferred through a piece of equipment using a more efficient lubricant, less power is being wasted to friction or heat. It is desirable for a lubricant to be optimized for maximum power throughput and to therefore allow for lower operating temperatures. Lower operating temperatures in gearboxes give rise to several benefits which include: lower energy consumption, longer machine life, and longer seal life. Seal failures are one of the principle reasons for repair and down-time in rotating equipment. A decrease of 10 degrees Celsius of operating temperature can double seal life and therefore decrease overall costs of operation and ownership.
A Small Worm Gear Rig measures both dynamic operating temperature and efficiency of power throughput simultaneously. In this gear rig, a splash lubricated bronze on steel worm gear set is the gearbox design employed. The subject worm drive gearbox, 1.75 inch centerline distance, 20:1 reduction ratio, was mounted in an L-shaped test rig with high precision torque meters on both the input and output shafts of the gearbox to measure power throughput efficiency performance based on control of output torque. The output torque was controlled to 100% of the rated load with a service factor of 1.0. Also, gearbox sump oil temperature was carefully monitored during operation using four thermocouples. National Basic Sensor located at 4921 Carver Avenue in Trevose, Pa. sells J-type thermocouples that are suitable for this rig test.
All torque and temperature data was logged every 10 seconds for a period of 12 hours after thermal stability was attained. The efficiency was calculated by establishing the ratio of output torque to input torque. The resulting efficiency (%) and operational temperatures (F.°) were compared for experimental blends to that of reference oils.
In addition to temperature & efficiency, air entrainment is another issue in lubricating oils. All lubricating oil systems contain some air. It can be found in four phases: free air, dissolved air, entrained air and foam. Free air is trapped in a system, such as an air pocket in a hydraulic line. Dissolved air is in solution with the oil and is not visible to the naked eye. Foam is a collection of closely packed bubbles surrounded by thin films of oil that collect on the surface of the oil.
Air entrainment is a small amount of air in the form of extremely small bubbles (generally less than 1 mm in diameter) dispersed throughout the bulk of the oil. Agitation of lubricating oil with air in equipment, such as bearings, couplings, gears, pumps, and oil return lines, may produce a dispersion of finely divided air bubbles in the oil. If the residence time in the reservoir is too short to allow the air bubbles to rise to the oil surface, a mixture of air and oil will circulate through the lubricating oil system. This may result in an inability to maintain oil pressure (particularly with centrifugal pumps), incomplete oil films in bearings and gears, and poor hydraulic system performance or failure. Air entrainment is treated differently than foam, and is most often a completely separate problem. A partial list of potential effects of air entrainment include: pump cavitation, spongy, erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shut down when low oil pressure switches trip, “micro-dieseling” due to ignition of the bubble sheath at the high temperatures generated by compressed air bubbles, safety problems in turbines if overspeed devices do not react quickly enough, and loss of head in centrifugal pumps.
Antifoamants, including silicone additives help produce smaller bubbles in the bulk of the oil. In stagnant systems, the combination of smaller bubbles and greater sheath density can cause serious air entrainment problems. Turbine oil systems with quiescent reservoirs of several thousand gallons may have air entrainment problems with as little as a half a part per million silicone.
One widely used method to test air release properties of petroleum oils is ASTM D3427-03. This test method measures the time for the entrained air content to fall to the relatively low value of 0.2% under a standardized set of test conditions and hence permits the comparison of the ability of oils to separate entrained air under conditions where a separation time is available. The significance of this test method has not been fully established. However, entrained air can cause sponginess and lack of sensitivity of the control of turbine and hydraulic systems. This test may not be suitable for ranking oils in applications where residence times are short and gas contents are high.
In the ASTM D3427 method, compressed air is blown through the test oil, which has been heated to a temperature of 25, 50, or 75° C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is usually recorded as the air release time.
In addition, there is a need for low temperature air release properties. Today, approximately, 80 percent of the gas turbines in North America operate in a cyclical mode or regular stop-start operation versus continuous base operation. Turbine oil with superior air release enables smooth fast starts by eliminating the potential for cavitations in the turbine hydraulic circuit. Turbines and compressor machinery are being designed to have a smaller equipment footprint for lower hardware and infrastructure costs. Consequently, units will operate at higher flow rates and with shorter residence times, demanding oils with improved air release. Air entrainment may accelerate oil degradation due to micro dieseling (adaptation compression) and/or oxidation mechanisms.
Due to the high energy prices, the energy and power industries are building facilities in more remote places including colder climates, such as Russia, which were previously not commercially attractive. Superior low temperature fluidity, pumpability and air release lubricant properties are advantageous for these applications.
Accordingly, there is a need for a lubricant that provides a consistent favorable operating temperature and power efficiency along with air release properties (including low temperatures) using high viscosity base stock blends. The present invention satisfies this need by providing a novel combination of base stocks that give the desired performance.